1. Field of the Invention
The present invention relates to a production method of an oxidation catalyst device for exhaust gas purification which purifies an exhaust gas from an internal combustion engine by oxidizing particulates contained therein with a catalyst composed of a composite metal oxide.
2. Description of the Related Art
Conventionally, there has been known an oxidation catalyst device for exhaust gas purification disclosed in, for example, Japanese Patent Laid-open No. 2002-66338, to purify an exhaust gas from an internal combustion engine by oxidizing particulates or hydrocarbons contained therein. The oxidation catalyst device includes a porous filter carrier having a plurality of cells formed from a plurality of perforated galleries which penetrate through the porous filter carrier in the axial direction and a boundary of each cell as a cell division, and an oxidation catalyst coated on the cell division.
It is known that a perovskite composite metal oxide having a general formula of AMO3 is used as the oxidation catalyst. In the general formula, the component A is at least one metal selected from a group of La, Y, Dy and Nd; and the component M is at least one metal selected from a group of Mn, Fe and Co. The perovskite composite metal oxide is prepared by substituting a part of the component A with at least one metal selected from a group of, for example, Sr, Ba and Mg. As a specific example of the perovskite composite metal oxide, La1-xSrxFeO3, La1-xBaxFeO3 and so forth may be given.
The oxidation catalyst device for exhaust gas purification may be produced in the following method: an organic complex prepared from a metal salt of each metal of the components A and M and an organic acid is formed as a catalyst precursor, a slurry containing the catalyst precursor is coated on a cell division of the porous filter carrier, and calcining the porous filter carrier to produce the oxidation catalyst device. Herein, the organic complex, as disclosed in, for example, Japanese Patent Laid-open No. 2007-237012, can be prepared by mixing an organic acid which may form the organic complex, such as sodium citrate, sodium maliate, sodium ethylene diamine tetra-acetate or the like, with a metal salt of each metal of the components A and M in a desired stoichiometric ratio and adding water.
However, in the oxidation catalyst device for exhaust gas purification mentioned above, there is a problem that it is difficult to raise a combustion performance of the particulates at a temperature below 300° C.
The porous filter carrier may be, for example, a rectangular one with a plurality of perforated galleries penetrated through the porous filter carrier in the axial direction disposed in a latticed pattern when viewed in profile. Accordingly, the coating of the slurry can be performed by streaming the slurry in the perforated gallery of the porous filter carrier from an opening portion thereof.
According to the oxidation catalyst device for exhaust gas purification produced in the above-mentioned method, it is conceivable that the combustion temperature at which the particulates in the exhaust gas from the internal combustion engine are combusted can be lowered. However, it is desirable to further lower the combustion temperature of the particulates.
Accordingly, there is disclosed in, for example, Japanese Patent Laid-open No. 2007-237012, an art to lower the combustion temperature of the particulates by forming not only a catalyst layer on the surface of the porous filter carrier but also on a wall surface of an air pore formed in the porous filter carrier. In the mentioned patent document, there is described a method for entering the slurry inside an air pore by sucking the slurry inside the perforated gallery from the opening portion thereof so as to form the catalyst layer on the wall surface of the air pore formed in the porous filter carrier. According to this method, a catalyst layer can be formed not only on the surface of the porous filter carrier but also on the wall surface of the air pore formed in the porous filter carrier. According thereto, the combustion temperature of the particulates can be lowered.
According to the method, the slurry is coated inside the air pore by sucking, therefore, a special device or equipment for sucking the slurry will be needed, leading to a problem of increased manufacture cost.